backflow

noun
  1. The reverse flow of a fluid, typically undesirable, in a pipe or vessel, often due to a pressure differential. noun
    The backflow of sewage into the basement was caused by a blocked drain.
    To prevent backflow, a check valve was installed in the plumbing system.
  2. A situation in which water or other liquids flow in the opposite direction from what is intended, potentially causing contamination or damage. noun
    Backflow in the irrigation system led to contamination of the clean water supply.
    The technician explained that a backflow preventer is necessary to avoid contamination of potable water.